During transpedicular instrumentation of vertebral column segments, the insertion of surgical implant close to sensible structures requires a very high degree of precision. For protection of nerves in proximity and blood vessels a high number of control X-ray images is acquired causing an increased irradiation. Despite multiplanar X-ray control there is a relatively high rate of misplaced implants caused by the difficulty of deducing 3D information from the acquired images and by the freehand drilling. According to a meta analysis by Kosmospoulos and Schizas[1] taking into account 130 ex- and in-vivo studies regarding accuracy of pedicle screw placements there is a variance of 0%-72% (median 10%) of implantation failure rate using conventional technique.
For better control of implantation and in order to avoid perforations, a multitude of computer assisted navigation and robotic systems especially in the domain of spine surgery have been developed and commercialized by research laboratories and by the industry. For spine surgery, especially in minimally invasive procedures, most computer assisted surgery systems use medical images for input patient data. From a methodological point of view these systems can be classified by the image modality (preoperative Computer Tomography (CT), intraoperative 2D, respectively 3D, fluoroscopy) and by the registration method of transfer of the planning into the operating site (navigated or robotically). Depending on the underlying principle, the surgical workflow as well as the advantages and disadvantages resulting from the respective boundary conditions change with respect to the conventional technique.
Generally speaking, computer assisted systems could prove in the framework of clinical studies, that the failure implantation rate of pedicle screws can be reduced significantly to 0%-28% (Median 5%) with respect to conventional approach[1]. Additionally, Grützner et al.[2] demonstrated in the framework of a clinical study, that by use of fluoroscopic navigation systems (2D or 3D) the irradiation dose could be reduced by up to 40% respectively 70%. Especially the Operating Room staff took advantage from this reduction besides the patient, the former being exposed to such irradiation on a daily basis during such interventions.
This positive tendency is not valid for CT based systems though, for which the overall irradiation balance for the patient is disadvantageous with respect to the conventional approach because of the navigation data set that needs to be acquired additionally to the diagnosis CT data set[3]. Additionally there are supplemental costs for the CT data such that a CT based planning is only justifiable in the case where the structures to be treated show large deformations.
The necessary detail accuracy of the data sets is with few concessions also provided with intra operative 3D imaging or 3D Fluoroscopy. These systems (e.g. Siemens Arcadis Orbic) allow the navigation within multiplanar reconstructions but with reduced quality and especially with a reduced scan volume (approximatively 12 cm×12 cm×12 cm) with respect to pre operative CT data sets. A major advantage of the intraoperative 3D imaging however is, that the datasets are acquired intra operatively just before the implantation and that the registration can be done automatically. Thanks to this, the probability of anatomic alteration (e.g in the case of traumatological interventions) between the preoperative CT scan and the Operating Room as well as registration errors can be minimized. This is also potentially reflected when comparing the position failure rates of such systems (4%-9% CT based[4]-[7], under 1% 3D fluoroscopy[8]-[9]).
There are several advantages and disadvantages of the CT based and the 2D and 3D fluoroscopic navigation systems that are being discussed controversial in the literature. In more detail these issues are:                Operating Room time compared to the conventional approach        The invasiveness necessary for the intervention (size/type of incision, attachment of reference basis to the bone, etc.)        Problems with the clinical and surgical workflow being modified in a different way        Purchase cost and additional cost per intervention        
An important limiting factor of navigation based systems is the necessary tracking system (mostly optical tracking) by which the registration (alignment of the planning data with the patient anatomy) as well as the positioning and alignment of the implantation instruments is performed. On the one hand the intra operative flexibility is greatly reduced by the “line of sight” problem and the limited work space, on the other hand the achievable accuracy is limited for example because of markers being soiled by blood or the temperature sensibility of the sensor system. Furthermore there is the problem of free hand positioning of the instruments (drills, drill guides, cutting jigs) which causes the results being heavily dependent on the dexterity of the surgeon besides exact planning. There is no controversy that the expenses for navigation bases systems are significant when compared with the conventional approach. The necessary purchase of a tracking system (costs between 10000 and 40000) is in the centre of focus here. There are additional costs for the different instruments (interfaces for the trackers on the instruments and guides, calibration tools, etc.) that are customized to the respective tracking based navigation system and for the costs for single use items necessary for each surgery (i.e. 500-1000).
A system being used for spine surgery in a clinical setting that does not require a tracking system neither for registration (automatic image based registration) nor for instrument alignment, is the semi active robotic assistance system SpineAssist® (Mazor Surgical Technologies, Caesarea, Israel)[10], also disclosed in WO 03/1009768. The system is attached to a reference basis that itself is attached dorsally to several segments of the vertebral column. This allows a robotically alignment of a drill guide in the direction of pedicle screw placement. After planning controlled alignment the robot is being switched off and the surgeon performs the drilling through the positioned drill guide. The system is based on pre operative CT data sets with the known advantages and disadvantages (except for the problems with registration). The alignment or so called registration between planning CT data set and the patient anatomy is carried out in a pure image based way by using biplanar fluoroscopic data sets (so-called “fluoromerge”)[11] and calibration fiducials being integrated in the reference basis.
The Robodoc (see U.S. Pat. No. 5,806,518) is another robotic system that is used for surgical applications for hip and knee. Despite the advantages which result from the different functional principles described above with respect to tracking based free hand navigation one can summarizes the limitations of the system on the basis of the problems that are discussed generally in conjunction with robotic assisted surgery:                Purchase costs (e.g. SpineAssist® approx. 120.000 ) plus additional costs per case.        Safety related methodological efforts (e.g. redundant safety architecture), since active components are in touch with the patient.        The operative and technological effort for maintaining sterility of the semi active robotic system (cable based system with six motor-encoder units).        The application specific design (work space) of the robotic system comprising a specific kinematics with therefore designed electronics and drive unit which do not allow a universal application for different medical problems.        
Furthermore there are different approaches originating in stereotactic neurosurgery. These systems allow the adjustment of a trajectory (e.g. to target a certain area in the brain) based on a 3D image data set (e.g. CT data set). The coordinate system of the stereotaxy frame is aligned with the planning image data set either by a direct unambiguous link to the reference frame of the CT-gantry, or by taking advantage of the visibility of certain parts of the stereotactic frame in the image data set. For a patent on this topic see for example U.S. Pat. No. 4,706,665, which describes a purely passive positioning system. Some axis of the articulated stereotactic frames can be driven electrically such that it becomes similar to a robotic system (see US 2007/0055389). The alignment of the stereotactic frame (respectively of the robot) can also be performed with positioning sensor technology as described in EP 0 728 446. WO 01/78015 and especially WO 02/37935 describes a system, where based on multiplanar X-ray images a planning for an osteotomy (bone cut) or respectively an osteosynthesis (alignment of two bone fragments) is generated with computer assistance and then realised with a mechanical device. More precisely the computer system calculates the necessary adjustment parameters for realising the plan, which is then adjusted accordingly by the surgeon. But those systems require robotics device to make it fast and accurate or they require manual adjustment of several screws which is slow and prone to human errors.
In those applications where patient images are used as input data, one objective of the invention is to provide a solution that does not require a navigation system or a robotics system but that is fast and easy to adjust accurately guides and instruments.
For applications outside of spine surgery, it is possible to assist the surgeon by a computer-assisted surgery system without using medical images. This refers to navigation systems. In those cases, using a navigation system with an optical or magnetic tracking system makes sense since it generates 3D data instead of medical images. Such data are used for optimal positioning of instruments. In navigation systems, trackers are attached to patient anatomical structures such as bone for example, but also to instruments such as cutting and drilling guides. However, the precise adjustment of cutting guides or drilling guides with navigation systems is usually done manually, using the navigation system as a visual control, or using adjustment guides with screws that are time consuming and cumbersome, and they may require additional fixations.
Many devices used in conjunction with navigation systems use screws to adjust and finely tune the position of a surgical instrument. For instance, in U.S. Pat. No. 6,712,824, Millar uses a mechanism with three screws to adjust a cutting plane guide for knee surgery, but the screws must be adjusted manually which takes time. Similar principles can be found in EP 1 444 957 by Cusick, or US 2006/0235290 by Gabriel. Moreover the mechanical architecture is serial and it does not lock automatically to a given position when the screws are not turned, it is therefore necessary to use additional pins in the bone to fix the guide.
More complex architectures are using more than three screws in order to adjust cutting blocks. For instance, in EP 1 669 033, Lavallee uses a navigation system to adjust the position of several screws of a femoral cutting block but this process is not easy and it takes a long time.
The tracking technology of navigation systems can take multiple forms. It includes, but is not limited to optical active technology, with active infrared Light Emitting Diodes (LEDs) on trackers, optical passive technology (with passive retro-reflective markers on trackers), mechanical passive arms with encoders, accelerometers and gyrometers, or magnetic technology. Those tracking technologies are known as prior art of navigation systems for surgery. In this type of applications which does not use medical images, it is therefore necessary to propose adjustments devices and methods to make fast, easy and precise positioning of surgical instruments using a navigation system.
Referring to FIG. 1, the instrument 1 is any surgical instrument that has the following characteristics:                [A] The instrument 1 has a tracker 10 attached thereon so that it is tracked by the navigation system 2. The navigation system 2 comprises a camera 20 and a control unit 21 such as a computer with a screen.        [B] The instrument 1 is rigidly fixed to a solid 3 that is also tracked by the navigation system 2.        [C] The instrument has a fixed part 11 which is fixed to the solid 3 and a mobile part 12 which is mobile with respect to the fixed part 11.        [D] The position of the fixed part 11 with respect to the mobile part 12 can be adjusted by screws 13. The number of screws is independent of the invention.        
A tracker 30 is attached to the bone 3 or directly to the fixed part 11 of the instrument. It is used as a reference for collecting data points and surfaces with the navigation system. The target of cutting plane position is defined in a coordinate system attached to tracker 30.
A screwdriver 7 is used to adjust the instrument position with respect to the solid 3 in a target position. The target position of the instrument is supposed to be selected by the surgeon or set to default values with respect to anatomical landmarks digitized with the navigation system. The target position is represented by a geometric relationship M0 between the fixed part 11 of the instrument and its mobile part 12. By trivial calibration, the target position can be represented equivalently to a geometric relationship M1 between a tracker attached to the mobile part and a tracker attached to the fixed part or to the solid.
The problem is for the user to move several screws 13 independently to move the mobile part 12 until the geometric relationship between the mobile part tracker 10 and the solid tracker 30 matches M1 within a very low tolerance limit such as for instance 0.5 mm and 0.2°.
The manual adjustment of individual screws 13 takes a long time and it is difficult to converge towards a solution.
To help this process, for any initial position of the screws 13 and mobile part 12, the control unit 21 of the navigation system 2 can calculate the necessary screw differential adjustments DSi, for each screw 13i (where i is from 1 to N and N is the number of screws), which is necessary to bring the mobile part 12 to the target position. This is an easy calculation that only requires knowing the geometry of the screw placements with respect to the mobile and fixed parts and that is specific to each geometry. In a first step, the display of the navigation system can simply show the adjustments necessary DSi on each screw to the user such that the user follows the indications on the screen. While the screws 13 are manually adjusted, the values DSi are recalculated in real-time by the navigation system and the user can adjust the various screws accordingly.
However, this process remains long and complicated.
The present invention thus aims at providing an adjustment process that is short and simple in order to save intraoperative time and reduce the risk of failure, and an adjustment device suited for such a process.